I get these type of requests to improve a presentation a lot, first when there are budget issues, and now more recently when I say that coding my app consumes 100% of my time. Unfortunately, there are no quick fixes in good freelance design work:
Doing quick fixes will turn you into well, a quick fixer, you join anonymous army of freelance designers that do patch work and compete on price. If not you, we will find 1,000 alternatives exactly like you. The race to the bottom as Seth Godin would call it.
Fixing slides is the last step in the process, first comes understanding someone’s story. That is a big fixed cost investment that you need to put into every project, even if the draft slides look decent.
Presentation design work can only be really effective when you have the creative freedom (and budget) to tear up the entire draft design and pick your own consistent approach.
Quick fixes always need to be completed quickly, doing a lot of these projects means you always will be extinguishing emergency fires and never get around to doing your real work. It is more productive to be able to plan your work over a longer period of time. Quick fixes actually impacts the quality of your overall work.
Substandard work creates a self reinforcing loop: you will attract similar types of clients, and you are no longer proud of the work you show when someone asks you for recent design work that you did.
For bigger design firms there is a business model for quick fixes. If you can work offshore in countries with lower wage costs and a 12 hour time difference so you can work overnight. You need a large, flexible pool of people that can respond to sudden work loads. To make this work you probably have to work with larger clients that can guarantee a steady work flow (consulting firms etc.), and have some sort of subscription / retainer pricing model. This is an entirely different business from that of a 1-person freelance design agency that is trying to build a global quality micro brand.